Running ants when using masks

When you say “masking an object” do you mean drawing a selection (for example, with the Lasso tool), or do you mean painting in a layer mask?

Because the “marching ants” appear only when using one of the selection tools (Lasso, Magic Wand, Quick Selection, Select Subject…). If instead you are using a brush to paint in a layer mask, which is what is usually considered “masking,” then there will be no marching ants, because the selection is defined by the black and white areas of the mask.

That also applies to Quick Mask mode — the selection is defined by where you painted, not by marching ants. When you exit Quick Mask mode, the quick mask is automatically converted into a selection, and then you see marching ants.

Painting on a mask does not result in a selection, thus no marching ants. If you want to make a selection from it, ctl+click the mask.

Ctrl h (Cmd h) toggles the marching ants on and off. Or you can use View > Show > Selection Edges

I should add, that I'm using Windows 10